The present invention relates generally to an acoustic well logging method and system for examining the earth's subsurface formations surrounding a borehole and more particularly, it relates to an improved method and system for acoustic well logging to obtain a measure of formation anisotropy surrounding the borehole.
It is a well known practice to survey formations adjacent to and surrounding well boreholes by acoustic logging techniques in which acoustic signals are generated and received by means of a logging tool run through the well borehole. Conventional acoustic well logging techniques involve generating and recording of axis symmetric pressure waves. The recorded signals provide a measure of the compressional wave velocity in the earth's formations surrounding the borehole. Such techniques generally depend upon the generation and detection of pressure waves and the determination of the traveltime of the pressure waves between the source and the receiver or between spaced receivers. By these techniques, the velocity of compressional waves through the subterranean formation surrounding the borehole can be determined in order to characterize the formation. The conventional acoustic well logging systems generally include a logging sonde suitable to be suspended in the borehole, a source with the sodde for generating axisymmetric pressure waves in the borehole fluid and one or more receivers with the sonde and spaced apart from the source for detecting pressure waves in the borehole fluid. The pressure waves in the borehole fluid generated by the source are refracted into the earth's formation surrounding the borehole. They then propagate through a portion of the formation, are refracted back into the borehole fluid at a point adjacent to the receiver and are then detected. The ratio of the distance between the source and receiver to the time between the generation and detection of the pressure wave can yield the compressional wave velocity of the formation.
When a pressure wave generated in the borehole fluid reaches the borehole wall, it can produce a refracted compressional wave in the surrounding earth formation as described above. In addition, the pressure wave can also produce a refracted shear wave in the surrounding earth formation as well as guided waves which travel partly in the borehole fluid and partly in the formation adjacent the borehole. Part of the shear wave is refracted back into the borehole fluid in the form of a pressure wave and is detected by the receiver in the sonde. Guided waves are similarly detected by the receiver. Any wave that is one of the three types of waves detected by the receiver can be called an arrival; the pressure wave in the borehole flui which is caused by refraction of compressioaal waves in the formation, the compressional wave arrival; those caused by refraction of the shear waves in the formation, the shear wave arrivals; and those caused by guided waves, the guided wave arrivals. Thus, the signal generated by the receiver is a composite signal which includes the compressional wave arrivals, the shear wave arrivals and the guided wave arrivals. Compressional waves travel faster than shear waves and shear waves usually travel faster than the guided waves. Therefore, the composite signal generated by the receiver includes the compressional wave arrivals as the first arrivals, the shear wave arrivals generally as the second arrivals, and the guided wave arrivals generally as the last arrivals.
The conventional acoustic well logging source generates pressure waves symmetrical about the logging sonde axis. When suhh symmetrical pressure waves are refracted into the surrounding formation, the relative amplitudes of the refracted shear and compressional waves are such that it is often difficult to distinguish the later shear wave arrival from the earlier compressional wave arrival and from the reverberations in the borehole caused by the refraction of the pressure wave in the formation. Therefore, it is often difficult to use as a conventional symmetrical pressure wave source for logging shear wave velocity. More recently, full wave train acoustic well logging systems have been developed to obtain a meauure of formation shear and compressional wave velocities as shown by Parks, et al. in U.S. Pat. No. 4,562,557 and Ingram, et al., in U.S. Pat. No. 4,575,830. Parks and Ingram both describe a method for estimating or determining the velocity of various arrivals in composite signals recorded by a linear array of receivers.
Recent advances in direct shear wave acoustic well logging techniques have developed sources for producing asymmetric pressure aaves for logging shear wave velocity in formations surrounding the well borehole. Exemplary of such asymmetric pressure wave sources are described in South African Patent Application No. 823678 and in Canadian Patent No. 1152201 both of which are incorporated by reference herein.
However, the acoustic well logging art has only recently addressed itself to the possibility that formations surrounding the borehole through which the acoustic energy propagates can or may be anisotropic. Implicitly, the conventional acoustic well logging techniques previously discussed require that formations surrounding the well borehole be isotropic. Indeed, there are certain rock features which can cause anisotropy and are certain rock types which can exhibit intrinsic anisotropy. Consequently, assuming that a formation surrounding the well borehole is isotropic can lead to errors of serious magnitude.
The present invention provides a solution to a need to obtain a measure of formation anisotropy with an acoustic well logging apparatus. If zones of formation anisotropy are present and not properly taken into account, acoustic well logging data will be insufficient to adequately define formation properties surrounding the well borehole. The present invention provides a method and system for obtaining a measure of formation anisotropy surroundin a well borehole. Conventional surface seismic data is almost always processed and interpreted using techniques which assume isotropic wave propagation. Consequently, knowledge of formation anisotropy, such as can be obtained from the present invention, can be used to more accurately process and interpret such seismic data.